Acting on a tip-off, police found six puppies with scars on their abdomens at a rural property near the city of Medellin in northern Colombia, the Colombian Police said in a news release.

Ultrasound scans revealed bags of liquid heroin hidden inside the living animals. Police said traffickers planned to retrieve the drug once the dogs had passed customs abroad.

“The lust for money leads criminals to commit acts of great cruelty,” the police said.

The puppies were recovering after the heroin was removed by veterinarians.

Using animals for smuggling is unusual although drug smugglers often rely on couriers to swallow condoms full of drugs and one Colombian woman was once found with cocaine surgically implanted in her buttocks.

Colombia is the world’s largest producer of cocaine and the main supplier of heroin to the United States.